Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Mollusks, or Soft-bodied Animals
Class Gastropoda

The majority of modern Mollusks belong to the Class Gastropoda, comprising about 105,000 species. Most of them inhabit seas and oceans, but some live in freshwater bodies and on land.

The body of gastropods consists of a HEAD, a FOOT, and a visceral mass enclosed in a spirally coiled shell (Fig. 42). The head bears 1 or 2 pairs of tentacles, eyes, and the Mouth opening. The foot often has the form of a broad sole.

Using it, mollusks slowly crawl or attach themselves to various objects. The visceral mass of most gastropods projects above The Foot as a large, more or less spirally wound sac covered by the mantle. The shell comprises the apex, whorls, and aperture. The inner walls of the whorls closely adjoin each other and form a central columella.

Fig. 42 External appearance of the Roman snail:

1 — shell; 2 — respiratory pore; 3 — 2 pairs of tentacles; 4 — foot.

The respiratory Organs of most gastropods are represented by gills, whereas some freshwater snails, like terrestrial forms, breathe with the help of a lung.

In air-breathing forms, the function of the lung is performed by the mantle cavity, the walls of which are richly branched with Blood Vessels. It communicates with the external environment via a respiratory pore.

The Nervous system of gastropods contains 5 pairs of ganglia interconnected by connectives and commissures. Among the Sense Organs, the organs of Vision, Touch, chemical sense, Olfaction, and equilibrium are well developed.

The Digestive System includes the Oral Cavity, Pharynx, Esophagus, Stomach, intestine, and rectum. Salivary Glands open into the pharynx, and many species possess a radula. The esophagus often expands into a crop. The ducts of the digestive gland (Liver) open into The Stomach. The intestine forms several loops. The rectum opens with the anus.

The Circulatory system is open. The Structure of the gastropod Heart is similar to that of bivalves.

Excretory organs are typically represented by 1 Kidney, its excretory pore opening into the mantle cavity. In pulmonate mollusks, due to the transformation of the mantle cavity into a lung, the excretory pore is located near the respiratory pore and opens to the outside.

Among gastropods, a significant number of species are dioecious, but hermaphrodites are also common. Fertilization is internal. Development in marine species proceeds with the alternation of 2 larval stages, whereas in many freshwater and terrestrial mollusks development is direct: a tiny mollusk resembling the adult emerges from the egg. Gastropods play a major role in nature and for humans. Many bottom-dwelling species feed on organic debris, accelerating its decomposition. Humans consume certain gastropods as food, such as the edible Roman snail, the whelk, the periwinkle, and others.

The Roman snail is found in Ukraine (south of Kyiv) and in southern Russia (the Caucasus, Crimea), where it damages vineyards by feeding on young shoots. Upon the onset of dry weather, the snail enters summer estivation. During this process, the mollusk withdraws its body into the shell and secretes mucus that hardens in the air, sealing the shell aperture. In this motionless state, the animal's life processes slow down. The snail can remain in such torpor for several months until favorable humid weather returns. For the winter, these mollusks burrow into loose soil. Roman snails are hermaphrodites, but reproduce only under conditions of cross-fertilization. They lay eggs in the soil.

In the temperate zone, snails with spirally coiled shells can be found on plants. These species are closely related to the Roman snail.

Among gastropods, slugs cause significant damage to agricultural crops. They have an elongated body; their shell is completely or partially lost, and the foot is well developed. By day, these mollusks hide under stones, boards, and heaps of various debris. They are active in damp weather and at night. In many regions of our country, the small (3–6 cm) grayish field slug is common as a dangerous pest of numerous agricultural crops. It devours winter crop shoots and damages potatoes, cabbage, beets, and other garden plants. The large (up to 15 cm) reddish-brown forest slug is found in forests. During reproduction, slugs lay eggs in secluded, damp places. To combat these harmful mollusks, tobacco dust mixed with lime and the spraying of plants with toxic chemicals are used. Mineral fertilizers are also lethal to slugs. Toads destroy many of these pests.

In Russia, about two dozen species of gastropods are found that breathe atmospheric air yet inhabit ponds, lakes, marshes, and slow-flowing rivers. These include pond snails (Lymnaea) and ramshorn snails (see Fig. 43). The large staghorn pond snail, which has a conical shell tapering toward the apex, is frequently found in stagnant Water bodies, alongside the small pond snail, which is about 1 cm long. This mollusk occasionally crawls onto the shore or onto plants. Ramshorn snails have a flat, coiled shell rather than a pointed one like pond snails.

Fig. 43 Shells of freshwater gastropods:

1 — great pond snail; 2 — auricular pond snail; 3 — oval pond snail; 4 — marsh pond snail; 5 — Physa; 6 — Bithynia; 7, 8 — river snails (Viviparus); 9 — keeled ramshorn; 10 — great ramshorn.

Pond snails and ramshorn snails easily adapt to aquariums, where one can observe their lifestyle and study their external Morphology.

Fairly large river snails inhabit lakes and ponds with clean water. Their shell is not pointed at the apex like that of pond snails, but is more rounded. The shell aperture is closed by a rounded operculum. They breathe by means of gills, and therefore, unlike pond and ramshorn snails, they do not rise to the water surface. In an aquarium, river snails require more careful maintenance because they need oxygen-rich water. River snails are dioecious. Females give birth to tiny snails.

Some gastropods (e.g., slugs, the small pond snail, Bithynia) serve as intermediate hosts for many parasitic worms (particularly flukes), among which are numerous causative agents of dangerous diseases in humans, poultry, and livestock.